1. Field
This patent specification relates in general to an optical recording medium, and more particularly to a phase-change recording medium provided with improved dielectric protective layers and reflective layers, feasible for implementing record/readout operations at high recording velocities and attaining desirable overwrite characteristics and storage durability, among others.
2. Discussion of the Background
Optical information recording media have recently come into use as viable information data storage and archival devices of large capacity. With a trend of versatile use of the optical recording media, it is a pressing need to acquire improved capabilities in recording velocity, density and capacity for recording information data.
Of the optical recording devices, a phase-change recording medium is now known capable of implementing repeated record/readout operations by means of laser beam irradiation utilizing phase transition between amorphous and crystalline states. For this type of recording media in particular, overwrite operations can be carried out using a single light beam and a relatively simple optical system for readout steps, which is advantageous over other memory devices.
A phase-change recording medium includes at least a supporting substrate and several contiguous layers formed thereon such as dielectric protective layers, a recording layer, and a reflective layer.
In a phase-change recording medium with such a construction, recorded data bits are formed generally by transforming the portions of the recording layer into the amorphous state, while erasure of the recorded bits is carried out by crystallizing these portions.
With the pressing needs described earlier, there has been achieved a storage capacity of 4.7 GB with a rewritable phase-change recording medium of 120 mm in diameter. With this trend, the capacity on one side of the recording medium may presumably increase to 15 by adopting decreased wavelengths of laser beam from a blue laser device, for example, and using an objective lens with a numerical aperture (NA) of 0.60. The performance of blue laser devices has been improving considerably as a practical light source in the blue wavelength range, assuring high output power at 400 nm, for example.
The trend toward higher data capacity and recording density is thus considered prevalent for phase-change recording media. In implementing high recording speed while still retaining such high density recording, phase-change recording media have to have the capability of achieving repetitive heating, quenching and annealing operations. As a result, it may become increasingly difficult for the recording media to retain high repeatability in overwrite cycles.
In contrast to the current nominal (1×) DVD velocity of 3.5 m/sec, the data recording at a velocity of 10 m/sec or more may be required in the near future, since the phase-change recording media are adopted not only for external memory of computers but also for video applications with the commencement of digital broadcasts, thereby necessitating high speed transfer of large volume of image data.
In order to achieve such high recording speed and high density recording, it is necessary to improve the materials for forming, and the construction of recording media, as well as to improve recording and readout systems and technologies.
A recording material utilized so far is a phase-change type material containing Ag, In, Sb and Te, as major ingredients, thereby achieving a rewritable recording media practically available with a capability of linear readout speed of 3.5 m/sec and a storage capacity of 4.7 GB. This material and media construction is not sufficient from the viewpoint mentioned just above, and further improvements thereof are yet to be made.
For example, the phase-change recording medium generally includes at least a ZnS—SiO2 dielectric protective layer, a recording layer containing Ag, In, Sb and Te, as the major components, another ZnS—SiO2 dielectric protective layer, and a reflective layer of Al alloys. With the present structure of the recording medium, however, its characteristics are not always satisfactory for achieving data recording at high recording speed and high bit density.
Although Ag metal may be considered as an alternative for its advantage such as, for example, high thermal conductivity, it also has a disadvantage in that it can easily deteriorate disc properties, for example, by sulfide formation with S elements included in the dielectric layer.
In addition, although substitutes for the ZnS—SiO2 material may be contemplated, no fully satisfactory replacements have been found so far.
Japanese Laid-Open Patent Application No. 2001-34043 discusses a double-layered structure of the dielectric protective layer, comprising a first layer of ZnS—SiO2 formed contiguous to the recording layer and a second layer of SiC formed contiguously to Ag reflective layer, whereby a recording medium has been formed having a large number of repetitive recording cycles. It is noted herein that the SiC material has a thermal conductivity and a melting point both higher than ZnS—SiO2, and a small difference in thermal expansion coefficient relative to either ZnS—SiO2 dielectric or Ag reflective layer.
Therefore, further improvements are highly desirable in the materials for appropriately forming the recording layers and dielectric protective layers for achieving recording at higher linear recording velocity (e.g., 10 m/sec or larger), and for improved storage durability.
In a phase-change recording medium, as described earlier, data recording is carried out by transforming portions of the recording layer into the amorphous state, while erasure of the bits is carried out by crystallizing these portions. During the step of crystallization (i.e., either initialization or erasure), the recording layer is heated to temperatures higher than its crystallization temperature but lower than melting point, which is followed by annealing at a speed slow enough for crystallization. In addition to the recording material, therefore, the overall structure of the recording medium is important to achieve suitable change in temperature with time in the recording medium.
Suitable materials for forming the dielectric protective layers preferably satisfy several requirements such as being sufficiently transparent in the wavelength range of the laser beam, having high melting, softening and decomposition temperatures, and being easily moldable.
These materials are therefore selected generally among metal oxides and nitrides, having satisfactory heat resistance and mechanical strengths.
The dielectric protective layers formed of such materials generally have large differences in thermal expansion coefficient and elastic modulus relative to the plastic substrate. As a result, the dielectric layer may be peeled from the substrate during repetitive recording and readout operations, thereby forming pinholes and cracks. In addition, the plastic substrate may be warped with relative ease from temperature and humidity.
Thus, there have been investigated a number of materials for forming the dielectric protective layers. Of these materials, the dielectric material such as ZnS, as major ingredient, further including SiO2 and Y2O3 has been discussed (for example, Japanese Laid-Open Patent Application No. 8-180458), and efforts have been made recently to further improve related materials.
These materials are known to have satisfactory adhesiveness to recording layer materials containing chalcogenide alloy such as GeTeSb, for example, which is advantageous for fabricating recording media.
During the step of transforming to the amorphous state (i.e., recording step), the recording layer is heated to temperatures higher than its melting point, which is followed by quenching, whereby recorded bits are formed. This step is suitably carried out by a heat dissipating layer, which is formed contiguously to the recording layer to also be used as a reflective layer (i.e., reflective/heat dissipating layer).
Examples of materials suitable for forming the reflective layer of optical recording media include Au and Al alloys, which have been in wide use, and Ag alloys which are used rather recently.
Although the Au layer is in use commercially for forming CD Recordable (CD-R) media for its advantageous properties such as, excellent chemical stability, high reflectivity, and thermal conductivity, it also has a disadvantage of relatively high costs.
In contrast, since Al alloys are less expensive and have relatively high reflectivity, they are in wide use for forming several types of optical recording discs such as CD (compact disc) and DVD (digital versatile disc), as well as MO (magneto optical disc) and CD-R (CD-rewritable).
Since the metal Ag has advantages such as reflectivity comparable to, and a thermal conductivity higher than Au, recording media provided with Ag reflective layer can be made. As a result, there may be achieved excellent initial disc characteristics useful for almost all types of optical recording media.
In addition, the Ag metal is less expensive than Au metal, and the velocity for forming Ag layer by sputtering method using Ag sputtering target is about three times higher than that using Al sputtering target.
The Ag metal, however, has several disadvantages such as less chemical stability, for example, thereby causing deterioration in media characteristics. This is caused by the effect of the materials included in neighboring layers, also by humidity infiltrated from the ambient.
As a result, reflectivity of the recording media decreases with time to the extent not to be able to carry out readout steps, and errors increase during readout steps, thereby decreasing overall reliability of the recording media.
In order to alleviate such difficulties, it is known that the Ag metal layer can generally be used in the form of Ag alloys including additives such as, for example, Pd (Palladium) and Rh (Rhodium). However, these additional metals are expensive.